a. Field of the Invention
The instant disclosure relates generally to propulsion mechanisms for an electric car, and more particularly to multiple electric motor-based mechanisms.
b. Background Art
It is known to use electric motors in electric or hybrid-electric vehicles. For example, it is known to use DC electric motors and/or AC electric induction motors in electric vehicle applications. However, there is desire to improve the efficiency of operation. With regard to the electric motor energization itself, various control schemes have been developed that improve efficiency of that aspect. For example, AC induction motor control approaches, such as variable frequency drive (VFD) technology, have improved efficiency. However, there remains fundamental inefficiency in the current electrical drive train architecture. The basic problems will be described below, in connection with FIGS. 16-17, which illustrate typical electric motor torque and power curves.
As shown in FIG. 16, the developed torque declines as the motor speed increases, such that the available torque at high motor speeds is relatively limited. As a consequence, as shown in FIG. 17, the available power also becomes relatively limited as the motor speed increases. Since the useful range of torque over speed (i.e., RPM) is limited, if an electric motor is directly coupled to a drive shaft without transmission, the vehicle will have a limited top speed and has limited torque and horsepower at such high speed. The typical solution to this problem is to incorporate a transmission between the motor and the drive shall. The transmission allows (i.e., through a gear ratio selection) a desirable motor RPM range, a range at which the motor can produce adequate torque and horsepower (and at a desired efficiency), can be associated with the desired vehicle speed.
One practical problem is that a transmission (e.g., automatic transmission) is difficult to design to match the special torque curve of an electric motor. Also, a transmission adds weight, cost, and efficiency loss to the vehicle and/or drive train. Moreover, the transmission has its own unique failure modes, which potentially affects reliability.
Conventional electric car drive train architecture is inherited from that used in internal combustion (IC) engine powered vehicle designs, namely, an architecture including one engine paired with one transmission. While hybrid electric vehicles add a supplementary source of power (i.e., both IC engine and electric motor), a transmission is still used to match the optimum RPM range of both power sources to a desired vehicle speed. Even with the use of emerging motor control approaches, such as VFD motor technology, electrical motors are nonetheless relatively inefficient at both low and high speed (RPM) and are also relatively inefficient at low power output levels. These limitations reduce the overall energy efficiency of an electric vehicle, which in turn reduces the effective driving range of such an electrical vehicle (per charge).
There is a need for an improved mechanical drive train architecture for an electrical vehicle that minimizes or eliminates one or more of the problems set forth above.